With respect to related prior art, reference is made to the current assignee's Finnish Patent Application No. 942451 (corresponding to U.S. Pat. No. 5,732,902, the specification of which is incorporated herein by reference) which describes a method and device for winding a web. In the method, the web is wound onto a roll spool on support of a support roll and through a nip formed between the support roll and the roll being produced. The spool is supported at least partly by means of a support member arranged in the center of the spool. The spool and the roll are supported and/or loaded by means of a device whose position can be varied. In the initial stages of winding, the loading and support units of the device are shifted substantially in a plane passing through the axes of the support roll and the roll being produced in order to load and/or support the roll being produced in the winding position. When the winding makes progress, the loading and support units of the device are shifted downwards along a path substantially parallel to the circumference of the roll, and in the final stages of winding, the roll that is being completed is supported by means of the loading and support units from underneath. By means of the invention described in Finnish Patent Application No. 942451, it is possible to wind rolls having a large size without faults, i.e., rolls having a diameter of even more than about 1.5 meters and a width of even more than about 3 meters.
However, in all center-drive winders, dishing of the roll occurs, in particular with larger roll diameters. Dishing is understood as a fault of the shape of the roll which arises from the fact that the web layers on the roll are shifted during winding in the axial direction of the roll (laterally). Owing to this effect, the shape of the ends of the roll take a form different from a plane shape, i.e., the ends of the roll become convex or concave, thereby causing an error in the shape of the roll. When such lateral shifting starts, it generally tends to intensify itself and ultimately has the consequence that the roll end becomes convex unless correcting operations are carried out early enough. The phenomenon arises from the fact that, between the surface layers of the roll, a slight extent of gliding always takes place during winding as a result of forces applied to the web in the nip. Owing to this gliding of the surface layers of the roll, the roll tightness is increased, and if these forces that increase the tightness are out of balance, for example, owing to uneven tension profile or thickness profile of the incoming web, the layers of web also tend to be shifted in the axial direction of the roll spool.
Also, faults in the alignment of the roll supports cause a similar error in the shape of the roll, and so does an uneven distribution of the nip force. Such an error in the shape of a roll is undesirable because of the problems that arise during unwinding of the roll.